Certain industrial processes depend on well-controlled flows of gas. One example is in the field of semiconductor device manufacturing, which uses a wide variety of gases for processing silicon wafers into integrated circuits (ICs).
Plasma etching is a particularly important semiconductor process that depends upon carefully controlled flows of a number of different gases. In plasma etching, various gases are introduced into a vacuum chamber. Electrical power (typically in the form of radio frequency excitation) is used to ignite a plasma that creates reactive gas species. The reactive gas species etch patterns into the silicon wafer to define different components of the IC.
Because of the extremely small dimensions of the components of modern ICs, effective manufacturing requires the use of gas flows exhibiting very stable and consistent mass flow characteristics. Conventionally, such mass flow is measured in standard cubic centimeters per minute (sccm).
Typically however, the electro-mechanical mass flow controllers (MFCs) used to control the flows of gases, are prone to drift over time. Semiconductor fabrication processes are especially sensitive to these drifts, since variations as small as a few percent can severely degrade the performance of the integrated circuit. Accordingly, maintenance of stable gas flows may require frequent testing and calibration of the mass flow controllers.
Conventionally, testing of the MFCs is accomplished by introducing the gas into a vacuum chamber of a known volume, while monitoring the pressure within that chamber. Based upon the known correlation between pressure, volume, and the mass of the gas introduced (which defines the number of molecules of the gas), the rise in pressure (“rate of rise”) as the gas flows into the vacuum chamber can be monitored. This information regarding pressure change within the chamber can then be used to determine the actual flow rate of gas through the mass flow controller.
For reasons of convenience, the vacuum chamber often used for the measurement of gas flows is the process chamber itself. The volume of the process chamber can be measured, for example, by monitoring a rise in pressure as gas is flowed through an MFC that is known to be accurate. Then, measurement of gas flow through any of the mass flow controllers connected to the process chamber can be readily accomplished.
One potential drawback of this conventional approach is loss in throughput of the process chamber. Specifically, the gas flow testing procedure consumes highly valuable time, during which no productive processing by the equipment can take place.
Another potential adverse consequence of this conventional approach is that deposits on the chamber walls from previous processing can serve to adsorb or desorb gases during the test. Where these deposits adsorb gases, the measured rate of the rise in pressure will be too low. Where the chamber deposits desorb gases, the rise in pressure will be too high. Either case will result in inaccuracies.
Moreover, even if there are no deposits present in the chamber, under certain conditions materials present on the walls of the chamber could adversely affect accuracy of the measurement. In one example, moisture on the walls of the chamber could react with a gas being flowed (such as silane), producing another gas (such as hydrogen) that throws off the pressure change and hence the flow rate calculation. In another example, ammonia bound to the chamber walls may react with TiCl4 flowed into the chamber, throwing off a flow rate calculation.
Still another potential disadvantage to the conventional approach for measuring gas flows is that any change to the volume of the process chamber will require another measurement of the chamber volume. For example, the addition or removal of a component such as a pressure gauge, can change the volume of the chamber, thereby causing the flow rate calculated from the rate of rise of pressure to be incorrect.
Certain approaches have been proposed in the past to deal with some of these issues. For example, a separate volume can be positioned upstream of the process chamber, where the rate of rise measurement can take place. Since this volume will not have the types of deposits present in the process chamber and since this volume will not change by having components removed from it or added to it, some of the disadvantages cited above are not present. This method, however, still requires a separate step during which no productive processing can occur, and there is the possibility of the gas reacting with adsorbed species on the volume wall present from a previous gas. A refinement of this approach includes a heat conductive assembly inside the volume for maintaining a constant temperature as the gas flows into or out of the volume. In one approach the volume already present within the mass flow controller is used as the known volume, instead of a separate container.
Yet another approach allows measurement of the gas flow while the gas continues to flow as a normal part of its process. In this approach, a known volume and a valve are positioned upstream of a gas flow controller that is maintaining a constant gas flow. Closure of the valve while the gas flow controller is maintaining a constant gas flow creates a pressure drop in the volume, where the rate of the pressure drop is proportional to the gas flow rate.
Although this allows measurement simultaneous with the gas flow controller going about its normal production use, it is limited to those applications where the change in pressure does not influence the operation of the gas flow controller. To avoid this problem, a pressure regulator may be installed upstream of the gas flow controller (or, as described below, upstream of a flow restriction) and downstream of a known volume and a valve to interrupt the gas flow. One of the disadvantages of such a solution is that the requirements on this pressure regulator are so rigorous that standard pressure regulators will not be adequate in this role. Although the function of a pressure regulator is to keep the downstream pressure constant while the upstream pressure can take on any value higher than the downstream pressure, in reality the downstream pressure is influenced by the upstream pressure. In addition, most regulators have some amount of hysteresis. Any change in pressure downstream of the pressure regulator will create errors in the measurement of the gas flow; consequently, these systems require highly sophisticated pressure regulators to work effectively.
A sophisticated pressure regulator may actually be part of a mass flow controller, which is composed of the pressure regulator, pressure transducer, and a flow restrictor used as a critical orifice. In this case, it makes sense to use a known volume and a valve arrangement to test the gas flow rate, since the pressure regulator is already in place. Most gas flow controllers in production use, however, such as the many mass flow controllers used in the processing of silicon wafers, do not contain such a pressure regulator as part of their design. Consequently, to test these mass flow controllers would require the addition of this sophisticated pressure regulator.
It is undoubtedly a result of these significant disadvantages that, for example, the semiconductor industry, which has great need for testing its mass flow controllers, has made only extremely limited use of these approaches.
FIG. 1 shows an embodiment of an apparatus 100 representative of the prior art. (See, e.g., U.S. Pat. No. 4,285,245 and U.S. Pat. No. 6,363,958). The apparatus comprises a gas line 101 having an inlet 103 in fluid communication with a gas source 104, and an outlet 105 in fluid communication with either a flow restrictor or mass flow controller. The pressure regulator 102 is used to establish a constant pressure of the gas flowing to the flow restrictor or mass flow controller. Under standard process conditions, the valve 106 would be open and gas would be flowing through the pressure regulator to the flow restrictor or mass flow controller, and then ultimately into the process chamber.
In FIG. 1, the volume V 110, represents the total fixed volume inside the pipes and other components present between the valve 106 and the gas flow controller (GFC), where the GFC can be, for example, a flow restrictor or mass flow controller (MFC). A pressure transducer 112 is configured to measure the pressure in the volume V 110 immediately upstream of the pressure regulator 102.
The function of the pressure regulator 102 is to maintain a constant pressure downstream of the regulator regardless of the pressure upstream of the regulator (as long as the upstream pressure is equal to or larger than the downstream pressure). Under these conditions, there is no increase of decrease in the number of moles of gas between the pressure regulator and the flow restrictor or MFC. Consequently, the flow of gas out of the MFC or flow restrictor is equal to the flow of gas through the pressure regulator.
If valve 106 is closed, then since there is no gas entering or leaving the volume 110 from the left, any gas leaving the volume must flow through the pressure regulator 102, but since the flow through the pressure regulator is equal to the flow through the MFC or flow restrictor, the flow out of the volume is equal to the flow through the MFC or flow restrictor. Since the amount of gas leaving the volume 110 can be calculated from the rate of drop of pressure in the volume, such a calculation allows a determination of the flow rate through the flow restrictor or MFC.
Unfortunately, as Ollivier explains in U.S. Pat. No. 6,363,958, most pressure regulators cannot control the downstream pressure to the level of precision that is required for an effective implementation of this flow measurement system. If the downstream pressure is not sufficiently controlled, two significant errors can be introduced: (1) the flow of gas leaving the volume 110 will not be equal to the flow of gas through the MFC or flow restrictor, and (2) the flow of gas through the flow restrictor, which is proportional to the pressure upstream of the flow restrictor, will not be the desired value.
For further information the reader is directed to: U.S. Pat. No. 5,684,245 to Hinkle; U.S. Pat. No. 5,925,829 to Laragione, et al.; U.S. Pat. No. 6,948,508 and U.S. Pat. No. 7,136,767 to Shajii, et al.; U.S. Pat. No. 4,285,245 to Kennedy; and U.S. Pat. No. 6,363,958 to Ollivier.
From the above, it is seen that improved techniques for testing for gas flows through gas flow controllers are desired.
Preliminary, due to the multitude of arrangements discussed herein, it is helpful to define a convention when referring to various plumbing elements. As used herein, a valve is a plumbing element used to shut off or turn on the flow of fluid. The on/off actions may be manual or automatic using some control scheme. A metering valve is a plumbing element that is used to shut off and fully or partially turn on the flow of fluid. This is a similar metering valve to that used in home water plumbing, where the user may turn the flow to a desired level. The on/off and partial on actions may be manual or automatic using some control scheme. A pressure regulator is a plumbing element that automatically cuts off the flow of fluid at a certain pressure at its output. Pressure regulators react to the pressure on their output side, and close when the pressure in the plumbing reaches the designated level. Should the pressure come down (for example, if someone were to open a faucet, i.e., open a metering valve downstream of the regulator), the regulator then opens and allows flow until the pressure is brought back up to its desired level, which is typically referred to as the set point. A typical pressure regulator uses the outside air, i.e., atmosphere, as a reference to bring the output (i.e., downstream) pressure to the desired set point. It regulates not on the pressure difference between the inlet and outlet, but rather the pressure difference between the outlet and the atmosphere.